1. Field of the Invention
This invention relates to a cryogenic refrigerator.
2. Description of the Related Art
FIG. 25 shows the construction of a conventional cryogenic refrigerator as disclosed, for example, in the summary of the lectures presented at the 45th Spring Meeting of the Cryogenic Superconduction Society. The cryogenic refrigerator shown is a Gifford-MacMahon-cycle refrigerator. In the drawing, numeral 1 indicates helium, which serves as the working fluid; numeral 2 indicates an inlet valve for charging in the helium 1; and numeral 3 indicates an outlet valve for discharging the helium 1. Numeral 4 indicates a first-stage expansion space; numeral 5 indicates a first-stage displacer which moves the helium 1 through a reciprocating movement; and numeral 6 indicates a first-stage accumulator, which contains a first-stage accumulating material which consists, for example, of disc-shaped phosphor bronze gauzes that are stacked together and tiny lead balls. Numeral 7 indicates a first-stage seal, which prevents the helium 1 in the first-stage expansion space 4 from flowing through the periphery of the first-stage displacer 5. Numeral 8 indicates a first refrigerating stage which absorbs thermal energy from an object to be cooled (not shown); and numeral 9 indicates a first-stage cylinder.
Numeral 10 indicates a second-stage expansion chamber; numeral 11 indicates a second-stage displacer which moves the helium 1 through a reciprocating movement; and numeral 12 indicates a second-stage accumulator containing a second-stage accumulating material which consists, for example, of a particulate matter such as Ho1.5Er1.5Ru, Er3Ni, or GdRh. Numeral 13 indicates a second-stage seal, which prevents the helium 1 in the second-stage expansion chamber 10 from flowing through the periphery of the second-stage displacer 11. Numeral 14 indicates a second refrigerating stage, which absorbs thermal energy from the object to be cooled (not shown); and numeral 15 indicates a second-stage cylinder.
Numeral 16 indicates a motor for driving the displacers 5 and 11; numeral 17 indicates a drive shaft for transmitting the driving force of the motor 16; and numeral 18 indicates a crank for converting rotary motion to linear motion. Numeral 19 indicates a compressor for compressing the helium 1; numeral 20 indicates a high-pressure buffer tank for mitigating fluctuations in pressure at a higher pressure level; numeral 21 indicates a low-pressure buffer tank for mitigating fluctuations in pressure at a lower pressure level; and numeral 22 indicates a differential pressure retaining device for keeping constant a differential pressure between the higher and lower pressure levels. Numeral 23 indicates thermal energy Qa absorbed by the first refrigerating stage 8; and numeral 24 indicates thermal energy Qb absorbed by the second refrigerating stage 14.
Next, the operation of this apparatus will be described. FIG. 26 is a graph showing the P-V chart of this refrigerator. The vertical axis indicates the pressure P of the second-stage expansion chamber 10, and the horizontal axis indicates the volume V of the same. In the condition indicated at D in FIG. 26, the second-stage displacer 11 is at its lowest position, and since the inlet valve 2 is closed and the outlet valve 3 is open, the pressure in the second-stage expansion space 10 is at a low level (e.g., approximately 6 bar). During the process of D-A, the outlet valve 3 is closed and the inlet valve 2 is opened, so that the pressure is raised to a higher level (e.g., approximately 20 bar).
Next, during the process of A-B, the displacers 5 and 11 are moved upwards and, at the same time, the helium 1 at the higher pressure level is introduced from the compressor 19 to the expansion spaces 4 and 10 while being cooled as it passes through the accumulators 6 and 12. Through stationary operation a temperature gradient is developed in accumulators 6 and 12 respectively. For example, the temperature at the upper end of the first-stage accumulator 6 is 300 K., whereas that at the lower end thereof is 30 K.; and the temperature at the upper end of the second-stage accumulator 12 is 30 K., whereas that at the lower end thereof is approximately 4 K.. Accordingly, the helium 1 introduced into the first-stage expansion space 4 is cooled to approximately 30 K., and that introduced into the second-stage expansion space 10 is cooled to approximately 4 K.. (The accumulating material used in the second-stage accumulator 12 is a rare earth alloy or compound which exhibits large specific heat at 10 K. or less, such as Ho1.5Er1.5Ru, Er3Ni, or GdRh, which accumulating material, however, is very expensive, costing as much as 2,000 to 10,000 yen per gram. In spite of this high price, such a material is used because other accumulating materials such as lead or copper have rather small specific heat at a low temperature of approximately 10 K. or less, so that heat exchange cannot be carried out in the accumulators, and the temperature of 4 K. is not reached.) Since any high-temperature helium 1 allowed to flow into the second-stage expansion space 10 through the second-stage seal 13 will constitute a heat load, the second-stage seal 13 is precisely made so as to minimize leakage. Also, since the accumulators 6 and 12 are heated by the helium 1, they exhibit a temperature distribution higher than the initial one.
During the process of B-C, the inlet valve 2 is closed and the outlet valve 3 is opened. In this process, the helium 1 in the expansion spaces 4 and 10 is expanded to change from the high-pressure state to the low-pressure state. In the course of this expansion process, the helium 1 in the expansion space 4 absorbs thermal energy Qa 23 from the object to be cooled (not shown) through the first refrigerating stage 8. Similarly, the helium 1 in the expansion space 10 absorbs thermal energy Qb from the object to be cooled (not shown) through the second refrigerating stage 14. When the temperature at this time is such that the helium 1 can be regarded as ideal gas when used in an isothermal process, the thermal energy that can be absorbed is equal to the area of the P-V chart. If the temperature is as low as approximately 4 K., because of the change of thermal property values of the helium 1, the amount of thermal energy that can be absorbed is reduced to a level approximately 10% of the area of the P-V chart.
The helium 1 then cools the accumulators 6 and 12, and returns to the compressor 19. In the condition indicated at C in FIG. 26, the pressure level in the expansion spaces 4 and 10 has become low.
In the process of C-D, the displacers 5 and 11 move downwardly to discharge the helium 1 whose pressure level has been lowered. After cooling the accumulator 6 and 12, the helium 1 returns to the compressor 19. If, in this process, the helium 1 at the low temperature is allowed to leak through the second-stage seal 13, part of the helium 1 will flow away and not cool the accumulator 12, resulting in a heat loss. This is another reason why it is necessary to precisely work out the second-stage seal 13. In the process of B-D, the accumulators 6 and 12 are cooled to restore their temperature distribution at the cycle start.
In the conventional cryogenic refrigerator, constructed as described above, the therma energy Qb that can be absorbed is reduced due to the thermal properties of helium, resulting in a deterioration in refrigerating efficiency. Further, the rare earth alloy or compound used is very expensive, resulting in an increase in the cost of the refrigerator. In addition, the seals 7 and 13 of the displacer sections become worn after a long term operation, with the result that leakage allowing the helium 1 to flow into the expansion spaces 4 and 10 occurs, resulting in refrigerating efficiency and reliability deteriorating.